The present invention relates to an apparatus and method for alarming decrease in tire air-pressure. In particular, the present invention relates to an apparatus and method for alarming decrease in tire air-pressure which can suitably judge decrease in an internal pressure of a tire in a vehicle mounting a limited slip differential device (LSD) thereon so as to securely prevent an erroneous alarm from being issued.
Conventionally, there has been known a matter that a dynamic load radius of the tire becomes small and a rotational speed becomes faster in comparison with a tire having a normal pneumatic pressure when a pneumatic pressure of the tire is decreased. For example, in Japanese Unexamined Patent Publication No. 149119/1995, there has been proposed a method of detecting decrease in internal pressure on the basis of a relative difference of number of revolution of the tire. Further, since the rotational speed of the tire is influenced by turning, speed acceleration and deceleration, load, speed of a vehicle and the like, various devices are considered in order to remove the influences.
However, among the vehicle in recent years, in order to improve a running performance such as cornering performance, there is a vehicle in which a limited slip differential device is mounted in a differential gear of a driving shaft. The limited slip differential device is so arranged that a differential operation is limited until a differential torque is over a set value in view of its mechanism, and the right and left driving wheels are rotated at a uniform speed. Accordingly, since an influence of decrease in a pneumatic pressure is not reflected to the number of revolution, it is impossible to detect the pressure decrease by a relative comparison of the number of revolution.
Judged values (DEL values) are collected on a portion having a relation of horizontal axis =0, that is, a vertical axis (Y axis) while the differential operation is limited regardless of an existence of the pressure decrease and a position, when a pressure of a tire in a rear right tire RR or a front left tire FL is decreased and a pressure of a tire in a rear left tire RL or a front right tire FR is decreased as shown in FIG. 8 in which a reciprocal number 1/R of a turning radius R calculated from the driving shaft is set to a horizontal axis (X axis). Accordingly, the applicant of the present application has found that the pressure decrease can be easily judged by comparing the judged value in a horizontal portion a little apart from the vertical axis (refer to Japanese Unexamined Patent Publication No. 123911/1999). Therefore, the arrangement is made such that when a difference A or B between the judged value at a time of pressure decrease and the DEL value of the normal pneumatic pressure is over a certain range, it is judged that the pneumatic pressure of the certain tire is decreased, whereby an alarm is issued.
However, in a clutch plate type LSD or the like, there is a case that the limited slip differential torque is changed with the passage of time. For example, in the case where the limited slip differential torque is decreased (DTD) or increased (DTU) as shown in FIG. 9 in comparison with a time of setting the pneumatic pressure of the tire to the normal pneumatic pressure and initializing a pneumatic pressure alarm apparatus (at a normal time), there is a risk that the DEL value is changed even when the pneumatic pressure of the tire is normal, and an erroneous alarm is issued in case the difference DTD or DTU with respect to the DEL value of the normal pneumatic pressure DS becomes equal to or more than a certain range.
The present invention is made in view of the above circumstances, and an object there of is to provide an apparatus and method for alarming decrease in tire air-pressure which can suitably judge decrease in an internal pressure of a tire in a vehicle mounting a limited slip differential device (LSD) thereon so as to securely prevent an erroneous alarm from being generated.
In accordance with the present invention, there is provided an apparatus for alarming decrease in tire air-pressure which alarms decrease of internal pressure of a tire on the basis of a rotational information obtained by tires attached to a four-wheeled vehicle, comprising:
rotational information detecting means for detecting a rotational information of each of the tires;
memory means for storing the rotational information of each of the tires;
calculation processing means for calculating a reciprocal number of a turning radius from the rotational information of the tire attached to a driving shaft among the rotational information of the respective tires and calculating a judged value from the rotational information of the respective tires;
first judging means for comparing the judged value with a reference value at every regions of the turning radius on the basis of a relation between the reciprocal number of the turning radius and the judged valve so as to judge decrease in internal pressure; and
second judging means for canceling a judgement of decrease of internal pressure when a difference between the judged value in the right turning region and the judged value in the left turning region is different from a difference between a reference judged value in the right turning region and a reference judged value in the left turning region which are obtained at a time of initializing run.
Further, in accordance with the present invention, there is provided a method for alarming decrease in tire air-pressure which alarms decrease of internal pressure of a tire on the basis of a rotational information obtained by tires attached to a four-wheeled vehicle, comprising the steps of:
comparing a judged value with a reference value at every regions of a turning radius so as to judge decrease in internal pressure on the basis of a relation between a reciprocal number of the turning radius calculated from the rotational information of a tire attached to a driving shaft in the four-wheeled vehicle and a judged value calculated from the rotational information of the tires attached to the four-wheeled vehicle; and
canceling a judgement of decrease in internal pressure when a difference between the judged value in the right turning region and the judged value in the left turning region is different from a difference between a reference judged value in the right turning region and a reference judged value in the left turning region which are obtained at a time of initializing run.